Building Control for London Extensions 2026: Process, Costs & Inspections
Planning permission gets most of the attention, but building control is the approval that actually governs how your extension is built. It is mandatory for almost all extensions, it runs in parallel with construction, and missing a stage can cost you the completion certificate you will need when you sell.
Quick Answer
Building control approval is required for virtually all home extensions regardless of whether planning permission is needed. You choose between a Full Plans application (drawings checked before work starts, recommended) or a Building Notice (work can start within 2 days, less protection). London fees typically range from £800–£2,500 for domestic extensions depending on size and borough. The completion certificate issued after the final inspection is essential for selling the property.
£800–£2,500
Typical London fee
3–6 weeks
Full plans check
6–7 stages
Inspection stages
Check your specific property constraints
Free Property CheckWhat Building Control Is and Why It Is Required
Building control is the system that ensures building work complies with the Building Regulations — the statutory minimum standards for structural safety, fire safety, energy efficiency, ventilation, drainage, and accessibility that apply to all new construction and significant alterations in England.
Where planning permission controls whether you can build something and what it looks like from outside, building control governs how it is built: the strength of foundations, the thickness of insulation, how electricity is wired, how drainage is connected, and how the structure would perform in a fire. The two systems are entirely separate, run by different parts of the council, and both apply simultaneously.
Building control approval is required for:
- All single and double-storey rear and side extensions
- Loft conversions and dormer extensions
- Garage conversions to habitable space
- Basement extensions and underpinning
- Structural alterations (removing or modifying load-bearing walls)
- New bathrooms, en suites, or kitchens (subject to location and drainage changes)
Important: Building control is required even when no planning permission is needed. An extension permitted under permitted development still needs building regulations approval. The two approvals are independent of each other.
Small, non-habitable structures — a garden shed under 15m², a small greenhouse — are exempt from building regulations. But the moment work involves a structural element, habitable accommodation, drainage, or notifiable electrical work, an application is required.
Who Carries Out Building Control: LABC vs Registered Building Control Approvers
In England, building control work on domestic extensions can be carried out by one of two types of body — and this changed significantly after the Building Safety Act 2022.
Local Authority Building Control (LABC)
The building control department of your London borough. LABC is the default option and remains the most widely used for domestic work. Each London borough operates its own LABC service with its own fee schedule. For work in a conservation area or a listed building, using LABC is generally advisable because the inspectors have local knowledge and often liaise informally with the planning department.
Registered Building Control Approvers (RBCAs) — formerly Approved Inspectors
Private-sector building control bodies. Before the Building Safety Act 2022, these were known as Approved Inspectors. Since 6 April 2024, all private-sector building control businesses operating in England must be registered with the Building Safety Regulator (BSR). The registration requirement ensures private inspectors meet the same competence standards as LABC officers. In practice, RBCAs and LABC carry out the same duties and issue equivalent completion certificates.
Which should you use?
For most London domestic extensions, the choice between LABC and a registered private approver comes down to price, responsiveness, and familiarity with your area. LABC fees are set by the borough and published; RBCAs can quote competitively, and some offer faster plan checking turnaround. Both produce a legally equivalent completion certificate — the document you need when you sell.
One practical consideration: if your extension is next to a party wall or requires a build-over agreement with Thames Water, LABC can sometimes facilitate internal co-ordination with other council departments more quickly than a private approver working externally. Ask your architect which body they have worked with recently for similar projects in your borough.
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Full Plans Application vs Building Notice
There are two main routes for obtaining building regulations approval for a domestic extension. The right choice depends on your project, your contractor, and your risk tolerance.
Full Plans Application
You submit detailed architectural and structural drawings — typically including floor plans, sections, elevations, and a structural engineer's calculations — to the building control body before work begins. The plans are checked against the Building Regulations and either approved (with or without conditions) or rejected with required amendments.
Building Notice
You submit a simple notice form to the building control body. No plans are checked in advance. Work can begin two working days after the notice is received. The building control inspector visits at key stages, checks compliance on site, and issues the completion certificate once the final inspection passes.
Recommendation: For most London extensions, use the Full Plans route. The 3–6 week delay to have drawings checked is significantly less costly than discovering a compliance problem during construction — particularly for foundations, structural elements, or drainage, where remediation after the fact is disruptive and expensive.
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Check now — it's free →The Inspection Process: Stage by Stage
Whether you use Full Plans or Building Notice, the building control inspector visits the site at defined stages of the work. You are responsible for notifying the inspector before each stage is covered up. If you cover work before the inspector has seen it, they may require you to expose it again — or in serious cases, refuse to issue the completion certificate.
The standard inspection stages for a single-storey rear or side extension in London are:
Stage 1: Commencement
Notify the building control body before or at the start of ground works. This confirms the project is live and inspection scheduling begins. Some authorities accept this notification via an online portal; others require a phone call or email at least 48 hours before ground works start.
Stage 2: Foundations
The inspector visits after the foundation trench is excavated but before concrete is poured. They check depth (typically 1m–1.5m in London clay, deeper near trees), width, bearing capacity, and soil conditions. London's shrinkable clay subsoil is the most common reason inspectors request deeper foundations than originally specified. Do not pour concrete until the inspector has signed off this stage.
Stage 3: Damp-Proof Course (DPC) and Oversite
When the walls are built up to damp-proof course level and the ground floor oversite (the concrete slab or beam-and-block system beneath the floor) is being installed, the inspector checks that the DPC is correctly positioned and continuous, and that the oversite and any under-floor insulation meets the required U-value standard. Ground floor insulation must achieve 0.25 W/m²K or better under the 2026 building regulations.
Stage 4: Structural Steel, Wall Insulation, and First Floor (where applicable)
Before internal walls are plasterboarded, the inspector checks structural steelwork (RSJs, padstones, lintels), cavity wall insulation and wall-ties, any first-floor joists and associated structural elements. This is the pre-plaster stage — one of the most important inspections, because once walls are boarded, none of this work can be seen without destructive opening up. Cavity wall insulation must achieve 0.18 W/m²K or better.
Stage 5: Roof Structure and Insulation
When the roof structure is in place but before roof covering is applied, the inspector checks structural adequacy of the roof (rafters, purlins, ridge), ventilation to the roof void (cold roof), and for flat roofs, the insulation specification. Roof insulation must achieve 0.15 W/m²K or better. For flat roofs, the construction specification is checked against the risk of cold bridging and condensation.
Stage 6: Drainage
Before drainage is backfilled, the inspector checks pipe runs, gradient (falls), access points, and connection to the existing drainage system. New foul drainage from kitchens and bathrooms must maintain adequate gradient to prevent blockages. A water or air test may be required to confirm drainage is watertight before it is covered. This stage is often combined with the foundations stage if both are ready at the same visit.
Stage 7: Final / Completion Inspection
When all work is complete, the inspector conducts a comprehensive final check: internal finishes, window and door energy ratings, electrical installation (a Part P certificate or competent person self-certification is required), fire detection (smoke and heat detectors), ventilation (mechanical extract in bathrooms and kitchens), and any conditions attached to the plan approval. Once satisfied, the completion certificate is issued.
For double-storey extensions, basement extensions, and loft conversions, additional inspection stages apply — typically including party wall junctions, structural steel at upper levels, and fire separation between floors.
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Competent Person Schemes: Self-Certification Without a Building Control Visit
Some building work within an extension can be self-certified by specialist contractors who are registered with government-authorised Competent Person Schemes, without the need for a separate building control inspection or application. This is a separate route from the main building control application for the extension itself.
The most important competent person schemes for extension work are:
Notifiable electrical work in a domestic dwelling — which includes new circuits, extensions to circuits in kitchens and bathrooms, and consumer unit replacements — must either be inspected by building control or carried out by an electrician registered with a competent person scheme. A registered electrician self-certifies the work and provides a Part P certificate, which forms part of the evidence base for the completion certificate.
All gas work must be carried out by a Gas Safe registered engineer. This is a legal requirement, not just a competent person scheme. The Gas Safe engineer provides a benchmark certificate for any new gas installation, which is required at the final inspection.
Installation of unvented hot water systems, boilers, heat pumps, and other heating systems can be self-certified by plumbers registered with the APHC or BESCA competent person schemes, covering the relevant parts of Part G (sanitation and hot water safety) and Part J (combustion appliances) of the Building Regulations.
Replacement windows and doors installed within or as part of an extension can be self-certified by a FENSA or CERTASS registered installer, confirming compliance with the thermal performance requirements of Part L. Without scheme registration, replacement glazing requires a building control application.
When commissioning subcontractors for extension work, confirm at tender stage that electrical and heating contractors are registered with an appropriate competent person scheme. This avoids the need for supplementary building control inspections and simplifies the paperwork trail for the completion certificate.
The Completion Certificate: Why It Matters
The completion certificate is the document issued by the building control body after the final inspection confirms that all building work complies with the Building Regulations. It is formal, legally significant proof that the extension was inspected and found to meet the required standards at each stage.
Why solicitors ask for it
When you sell a property, the buyer's solicitor will raise standard enquiries about any extensions or structural alterations. They will ask to see the planning consent (or evidence of permitted development) and the building regulations completion certificate for every piece of work. A missing certificate is one of the most common reasons residential property sales are delayed or fall through.
Mortgage lenders are equally strict. Most high-street lenders will not lend against a property where an extension lacks a completion certificate, effectively removing from your buyer pool everyone who needs a mortgage. This means only cash buyers — typically a small minority — can proceed, and often only at a discounted price.
What to do if you are missing a completion certificate
If building work was carried out without a building control application, or if inspections were missed, a regularisation application can be made to the local authority retrospectively. The LABC will inspect the work (which may require opening up walls, floors, or ceilings to check hidden elements), assess compliance, and issue a regularisation certificate if the work is satisfactory. Regularisation is only available through LABC, not private registered approvers.
For work completed more than 12 months ago where enforcement action is now time-barred, indemnity insurance is a pragmatic solution at sale. The policy covers the buyer against enforcement action or the cost of remediation if the work is found to be non-compliant. Indemnity insurance typically costs £20–£300 and is arranged by your conveyancing solicitor. It does not confirm the work is compliant — it simply insures against the financial consequences if it is not.
If building control was carried out but you have lost the certificate, contact your London borough's building control department. Most authorities retain records and can issue a duplicate completion certificate on request, usually for a small administration fee. Applications made through a private approved inspector (now RBCA) may require you to contact the relevant business directly.
Keep your documentation. Store the completion certificate with your property deeds and ensure your solicitor has a copy on the title file. This single document will be requested at every future sale, remortgage, and equity release application.
Building Safety Act 2022: What Changed for Domestic Extensions
The Building Safety Act 2022 was primarily aimed at higher-risk residential buildings (broadly, multi-storey residential blocks over 18m). Its direct impact on standard domestic extensions in London is limited but worth understanding.
All private building control businesses in England had to register with the Building Safety Regulator before 6 April 2024 (extended to 6 July 2024). Any private inspector you engage for domestic work is now an RBCA — verify their registration on the BSR register before appointing them. The transition was largely seamless for domestic clients.
The Act tightened competence requirements for all building control professionals. LABC inspectors and private approvers alike must now demonstrate competence to national standards. For domestic homeowners, this means the inspectors carrying out your stage inspections are subject to stronger professional oversight than before.
The fundamental process for domestic extensions — Full Plans or Building Notice, stage inspections, completion certificate — is unchanged. The Building Safety Act did not alter the Building Regulations that apply to houses, and the inspection regime for domestic extensions is the same in 2026 as it was before the Act, albeit with better-regulated inspectors carrying it out.
Building Control Fees in London 2026
London boroughs charge the highest building control fees in England. The fee structure covers plan checking (for Full Plans applications) and site inspections. For a Building Notice, no plan check fee applies, but the inspection fee is the same or higher.
Fees vary by borough and by the size and complexity of the work. Typical 2026 fee ranges for London domestic extensions:
| Extension type | Building Notice fee | Full Plans fee |
|---|---|---|
| Single-storey extension (up to 40m²) | £800–£1,200 | £900–£1,400 |
| Single-storey extension (40–100m²) | £1,100–£1,600 | £1,200–£1,800 |
| Double-storey extension | £1,200–£1,900 | £1,400–£2,200 |
| Loft conversion (with dormer) | £1,000–£1,500 | £1,200–£1,800 |
| Basement extension | £1,500–£2,500+ | £1,800–£2,500+ |
Inner London boroughs — including Camden, Islington, Hackney, Southwark, and Kensington & Chelsea — tend to sit at the upper end of these ranges. Outer London boroughs are typically £100–£300 lower. Building control fees are subject to VAT.
What the fee covers
Note: 50% of the plan charge is typically paid with the application; the balance is due on commencement of work (Full Plans route). Building Notice fees are paid in full with the notice.
Common Reasons Building Control Applications Fail or Get Delayed
Inadequate structural drawings or missing calculations
For a Full Plans application, building control requires structural engineer's calculations for any new structural element — RSJ beams, padstones, lintels, columns, and new foundations. Applications submitted without calculations, or with drawings that do not match the structural engineer's specification, are returned for amendment. This is one of the most common causes of delay at plan check stage.
Failing the foundations inspection
London's clay subsoil is expansive — it shrinks and swells with moisture changes. The building control inspector may require deeper or wider foundations than the structural engineer's initial specification if ground conditions at the actual trench differ from assumed bearing capacity. This is not a failure of the application, but it delays the pour and can increase the groundworks cost by £1,000–£5,000 depending on depth required.
Insulation shortfalls
Building control inspectors increasingly check that the specified insulation products and thicknesses have actually been installed as specified. Swapping specified products for cheaper alternatives mid-build — a common practice among less scrupulous builders — can result in a failing inspection. The remediation of walls or floors to insert missing insulation after they are boarded or tiled is expensive and time-consuming.
Missing Part P certificate or competent person self-certification
At the final inspection, the building control officer will ask for the electrical installation certificate (Part P) for all new notifiable electrical work. If the electrician was not registered with a competent person scheme and no building control inspection of the electrical work was arranged, the final certificate cannot be issued until the electrical work is independently verified — which may require cutting into finished walls to check concealed cabling.
Drainage gradient problems
New foul drainage installed during an extension must maintain adequate fall (gradient) from the point of collection to the connection with the existing system. Flat or near-flat drainage runs are a frequent issue in rear extensions where the drain must travel a significant distance across the plot. If the drainage inspection reveals insufficient fall, the drain must be relaid before it can be covered — which, if landscaping has already been reinstated, means significant additional cost.
Frequently Asked Questions
Is building control the same as planning permission for a London extension?
No. Planning permission controls whether you can build and what it looks like. Building control governs how it is built — structural safety, insulation, fire safety, drainage, and so on. Both are required for most extensions. An extension built under permitted development (no planning permission required) still needs building regulations approval. The two systems are run by separate departments and operate independently.
What is the difference between a Full Plans application and a Building Notice?
A Full Plans application involves submitting detailed architectural and structural drawings for checking before work begins. Problems are identified at design stage before any ground works are done. A Building Notice allows work to start within 2 working days with no advance plan check — compliance is assessed by the inspector during site visits. For most London extensions, Full Plans is recommended: catching compliance issues at design stage is far less costly than discovering them during construction.
How much does building control cost for a London extension in 2026?
Typical London fees for a single-storey extension (up to 40m²) are £800–£1,400 depending on the borough and whether you use Building Notice or Full Plans. Double-storey extensions run £1,400–£2,200. Basement extensions are £1,800–£2,500+. Inner London boroughs charge more than outer London. Fees are subject to VAT. For comparison, outer London boroughs and areas outside London typically charge £400–£800 for the same work.
What is a completion certificate and why do I need it?
A completion certificate is issued by the building control body after the final inspection confirms all work meets the Building Regulations. It is formal legal proof that the extension was inspected and found compliant. When you sell, the buyer's solicitor will ask for it. Mortgage lenders require it for the property to be mortgageable. Missing it can stop a sale, require expensive indemnity insurance, or force you to apply for retrospective regularisation — so obtaining it at the end of the build is essential.
What is the difference between LABC and a Registered Building Control Approver?
LABC (Local Authority Building Control) is the building control department of your London borough. A Registered Building Control Approver (RBCA) is a private-sector building control body — the successor to what were previously called Approved Inspectors. Since April 2024, all private building control businesses must be registered with the Building Safety Regulator under the Building Safety Act 2022. Both issue legally equivalent completion certificates. RBCAs can sometimes offer faster plan checking; LABC has local knowledge and can liaise directly with planning and Thames Water.
Do I need to notify building control for an extension covered by permitted development?
Yes. Permitted development and building regulations approval are entirely separate. Permitted development determines whether you need planning permission. Building regulations approval is required regardless of planning status — it is the law that governs how the structure is built. You must apply for building regulations approval and follow the inspection process for any permitted development extension that involves structural work, habitable space, drainage, or notifiable electrical work.
Summary
Building control is a mandatory, ongoing process that runs alongside construction from first ground works to the day the completion certificate is issued. Unlike planning permission — which is resolved before a spade goes in the ground — building control involves real decisions at every stage of the build, and missed or failed inspections have direct cost consequences.
For London extensions, the Full Plans route is almost always the right choice for anything beyond the most straightforward project. It catches compliance issues at design stage, provides approved drawings for your builder to work from, and gives you legal protection if problems emerge later. Building Notice is faster to initiate but shifts compliance risk onto the construction phase — where discovery is more expensive.
Budget £800–£2,500 for building control fees depending on the borough and project scale. Keep every document — the plan approval, stage inspection notices, competent person certificates, and above all the completion certificate. These are not bureaucratic formalities: they are the evidence that your extension was built correctly, and you will need them every time the property changes hands.
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